This presentation covers how packet forwarding is done on the networking devices, how Cisco Express Forwarding (CEF) works and different ways to do static routing.
5 • When the packet comes in, the router does the following: • Checks and removes L2 header, gets destination IP • Routing process • Switching process • L2 encapsulation Packet forwarding
6 • Find the longest match based on destination IP in routing table (RIB) • The goal is to select outgoing interface and Next Hop IP address (if applicable) Routing process
8 • 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]] Routing table (cont.)
9 • Metric (maximum is 232-1 = 4294967295): • Used to choose the best route within a single routing protocol (there are exceptions) • Administrative distance (0..255): • Used to choose the best route between routing protocols Routing table (cont.)
13 • 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) L2 Encapsulation
14 • 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 Traffic types
15 • 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 Processors: CPU and ASIC
16 • 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 Processors: CPU and ASIC
18 • 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 Memory: RAM, CAM and TCAM
19 • 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) Memory: RAM, CAM and TCAM
20 • 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 Process-switching
21 • 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 Fast-switching
23 • 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) Cisco Express Forwarding (CEF)
25 • 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] CEF – Adjacency table
27 • 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 • TCAM stores not only FIB, but ACL and QoS rules, however the allocation is predefined • On some platforms you can change allocation profile CEF on hardware-based platforms
29 • 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 next-hop, depending on the traffic share count in RIB CEF – Load balancing
30 • Verification: • show ip cef [ip [mask]] internal – shows NH-to-bucket distribution • show ip cef exact-route – shows NH and interface for source/destination IP pair • Change load balancing method (not recommended): (config-if)# ip load-sharing per-packet CEF – Load balancing (cont.)
31 • 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]] CEF polarization
33 • 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] Static Routing
34 • 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 Static Routing (cont.)
35 • 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 Static Routing to next-hop
36 • 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 the 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 of the outgoing interface is up • Use only for point-to-point interfaces! Static Routing to outgoing interface
37 • 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 the outgoing interface is up Static Routing to outgoing interface and NH
38 • 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 has AD 1 and tracking (based on IP SLA) or BFD • Secondary static default route has AD 2 or higher • Once tracking object/BFD goes down, primary route is deleted from RIB and secondary route is installed Floating static routes
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? 39 Recursive lookup for static routes - exercise
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. 40 Recursive lookup for static routes - rule
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