routing basics/cef
TRANSCRIPT
Intro to routing and switching
Routing
• Finding the optimal way towards destination
Switching
• Moving packet between interfaces
2
Packet forwarding
When the packet comes in, the router does the following:
0. Checks and removes L2 header, gets destination IP
1. Routing process
2. Switching process
3. L2 encapsulation
3
Routing process
• Find the longest match based on destination IP in routing table (RIB)
• The goal is to find outgoing interface and Next Hop IP address (if applicable)
4
Routing table - example
5
Routing table (cont.)
• Recursive Lookup
• Contains useless information for forwarding:
• For example, Administrative Distance and Metric
• Single lookup has linear complexity O(n)
• Stored in RAM
• Verification: show ip route [ip-address [mask]]
6
Routing table (cont.)
Metric (maximum is 232-1 = 4294967295):
• Used to choose the best route within a single routing protocol
(*not always true)
Administrative distance (0..255):
• Used to choose the best route between routing protocols
7
Routing table (cont.) - AD
8
Route Source Value
Connected 0
Static 1
EIGRP summary 5
eBGP 20
EIGRP internal 90
IGRP 100
OSPF 110
IS-IS 115
Route Source Value
RIP 120
EGP 140
ODR 160
EIGRP external 170
iBGP 200
NHRP 250
DHCP learned 254
Unknown* (Not installed) 255
Routing protocols
• Static
• Dynamic:
• IGP:
• Distance-vector (RIP, EIGRP)
• Link-state (OSPF, IS-IS)
• EGP:
• Path-vector (BGP)
9
Switching process
• Process-switching
• Fast-switching
• Cisco Express Forwarding (CEF)
10
L2 Encapsulation
Knowing outgoing interface and Next Hop address is not always enough for “packet rewrite”
• Point-to-point links (PPP, HDLC) – no additional information required
• Point-to-multipoint links (Ethernet, Frame-relay, ATM) – L2 Destination Address is required (from ARP cache, Frame-Relay/ATM mappings)
11
Traffic types
Data plane – traffic through the device
Control plane – traffic to the device:
• Routing protocols hello/updates
• BPDU
• FHRP and others
Management plane – part of control plane:
• SSH/Telnet
• SNMP
12
Processors: CPU and ASIC
• Central processing unit (CPU) is the brains of the network device
• Handles control plane
• Can do anything
• Can’t do packet forwarding with high throughput*
*Note: DPDK project enables x86 multi-core processors to forward 200+ Gbps
13
Processors: CPU and ASIC
• Application specific integrated circuit (ASIC) is circuit with transistors
• Very fast, but dumb
• Designed specifically to move packets
• Expensive
• Not possible to program new features
• Responsible for data plane
14
Memory: RAM, CAM and TCAM
Random Access Memory (RAM) is the most common type of memory
• Value is accessed by pointer (memory address)
• Cheap
15
Memory: RAM, CAM and TCAM
Content-addressable memory (CAM)
• Value is accessed by a key, not a pointer
• Very fast
• Expensive
• High power consumption
• O(1) constant time lookup
• Used in switches for MAC address table
16
Memory: RAM, CAM and TCAM
Ternary Content-addressable memory (TCAM)
• Value is accessed by a key, which consists of not only “0” and “1”, but also “don’t care” bits.
• Very expensive
• High power consumption
• O(1) constant time lookup!
• Used for next-hop lookup (CEF table), ACL (security and QoS)
17
Process-switching
• Recursive lookup is performed by CPU in RIB
• There is special process responsible for process-switching “IP Input”
• The following traffic is process-switched:
• Control plane
• Locally generated (not all)
• No L2 adjacency information
• ACL logging
18
Fast-switching
• First packet for source-destination IP pair is process-switched
• IP pair and corresponding encapsulation information is added to the cache
• Following packets are forwarding based on the entry in cache
• Deprecated
19
Cisco Express Forwarding (CEF)
The idea is to precompute and optimize information in RIB:
• Resolve recursive lookup and get rid of useless information
• Add pointer to pre-built L2 header in Adjacency table
The new table is called Forwarding Information Base (FIB) or CEF table:
• Contains prefix, NH, outgoing interface, pointer to L2 header
• Stored in DRAM [O(1) using 256-way mtrie data structure] and TCAM [if exists, also O(1), but much faster]
The lookup is done during the interrupt (process scheduling is not required)
20
CEF (cont.)
Verification:
show ip cef [ip-address [mask]] [detail] [internal]
Shows NH, outgoing interface, MPLS labels (if applicable)
Internal keyword shows pointer to Adjacency entry and hash buckets
Disable CEF:
(config)# no ip cef
21
CEF – Adjacency table
• CEF process takes information from all L3-to-L2 mappings and builds L2 header
• Adjacency table contains NH, interface, associated L2 Header
• Stored in RAM
• Pitfall: CEF process does not allow adjacency to age out
(clear arp won’t delete ARP entry if it can be revalidated)
• Verification:
show adjacency [detail]
22
CEF – Adjacency types
• Cache
• Glean
• Receive
• Punt
• Null
• Discard
• Drop
23
CEF on hardware-based platforms
• CEF basically allows to forward traffic without CPU
• Depending on platform there can be zero, one or more ASICs.
• The same applies for TCAM
• All L3 switches have TCAM, only some routers have it
• That’s why generally speaking L3 switches forward traffic faster than routers
• TCAM stores not only FIB, but ACL and QoS rules, the allocation is predefined though
• On some platforms you can change allocation profile
24
CEF on hardware-based platforms (cont.)
25
RIB
ARP Cache
Other L2 information
FIB(CEF Table)
Adjacency table
TCAM
RAM
ASIC
CEF – Load balancing
• Routing protocols can install several routes for the same prefix
• How will CEF decide where to send packet?
• CEF is doing load-balancing per-flow
• By default, it takes source-destination IP pair, feeds it to the hashing algorithm, returns the number of the bucket
• Buckets are allocated automatically per NH, depending on the traffic share count in RIB
26
CEF – Load balancing (cont.)
Verification:show ip cef [ip [mask]] internal – shows NH-to-bucket distribution
show ip cef exact-route src-ip dst-ip – shows NH and interface for source/destination IP pair
Change load balancing method (not recommended):(config-if)# ip load-sharing per-packet
27
CEF polarization
• Hash algorithm is deterministic, meaning that for the same source/destination IP pair the bucket (outgoing link) is the same.
• Result is that some links can be underutilized (especially if we have chain of routers with ECMP).
• One possible solution is to include L4 ports in hashing (if supported):
(config)# ip cef load-sharing algorithm include-ports [source[destination]]
28
Static Routing
• The way to install an entry in RIB manually
• Usually overrides entries installed via dynamic routing protocols
• Advantage: gives full control over path selection in your network
• Main disadvantage: huge administrative burden
• Syntax:(config)# ip route prefix mask [NH-IP | interface [NH-IP]] [distance] [track track]
• Verification:# show ip route [static]
29
Static Routing (cont.)
There are three different ways to configure where the traffic should go for specific prefix:
• By specifying next-hop IP address
• By specifying outgoing interface
• By specifying both
30
Static Routing to next-hop
• Recursive lookup is required to find outgoing interface
• On multipoint interfaces resolution of next-hop IP address is required (ARP cache, Frame Relay/ATM mapping)
• Static route is installed into RIB only if recursive lookup is successful (outgoing interface was found)
• It will stay in RIB even if next-hop is covered only by valid default route
31
Static Routing to outgoing interface
• Recursive lookup is not required because we know outgoing interface
• On point-to-point interface we can send the packet right away
• On multipoint interface first we need to find L2 address for destination IP• For every new destination IP address in the packet we will install entry in ARP
cache
• It can still work if Proxy Arp is enabled (which is enabled by default in IOS)
• Static route is installed into RIB only if line protocol (for outgoing interface) is up
• Use only for point-to-point interfaces!
32
Static Routing to outgoing interface and NH
• Recursive lookup is not required because we know outgoing interface
• On point-to-point interface we can send the packet right away
• On multipoint interface first we need to find L2 address for next-hop IP address
• Static route is installed into RIB only if line protocol of outgoing interface is up
33
Floating static routes
• Floating static route is a route that has AD higher than default and is not installed into RIB under normal operation, because there is another preferred path
• Once primary path fails floating static route can be installed
• For example:
• Primary static default route with AD 1 and tracking (based on IP SLA) or BFD
• Secondary static default route with AD 2 or higher
• Once tracking object/BFD goes down, primary route is deleted from RIB and secondary route is installed
34
Recursive lookup for static routes - exercise
• R1 has one interface up/up with IP in subnet 188.1.12.0/24. You configure the following:
Question: Which static routes will be installed in RIB?
35
Recursive lookup for static routes - rule
Answer:
Rule:
If the best route for the next hop also covers the entire address space of the static route under the question, it will NOT be installed.
36
Additional Resources
• Inside Cisco IOS Software Architecture (Russ White) book
• IP Routing FAQ
• Switching Paths
• Load Balancing with CEF
• Troubleshooting load balancing with CEF
• CAM vs TCAM
• CEF polarization
37
Questions?
38