Cisco – Why Label switching is faster than table lookups

cisconetwork-corerouterswitch

I learned that Label Switching (LS) technique, such as MPLS, is really faster than the classical routing method where each router takes a bit time to look up its IP table for routing to the next hop. If this is the claim, I want to understand why label switching is faster? To me both techniques are still similar, in terms of time, because routers still need to check the label on each packet before switching!! Right? I don't see any less computation or speeding up here. If I am wrong, I'd like to know the correct reasoning of that.

Thank you

Best Answer

I agree - I don't think the claim that label switching is faster than IP routing is true (regardless of what the current "label switching" article on wikipedia says).

Maybe back in the day on platforms that did forwarding in software, IP routing look-ups might have been expensive and time-consuming when the routing table got bigger, but I don't see how there would be any difference today in a label lookup vs. a route lookup in a FIB implemented in ASIC.

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Cisco – route tags and policy routing alternatives – traffic engineering question

I am not 100% sure whether tags are carried in BGP updates but here is what I put together pretty quick:

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Router "Static":

interface Loopback0
 ip address 172.16.1.1 255.255.255.0
!
interface Loopback1
 ip address 172.16.2.1 255.255.255.0

interface Ethernet0/1
 ip address 172.16.0.1 255.255.255.0
 half-duplex

ip route 0.0.0.0 0.0.0.0 172.16.0.2

Router R1:

interface Ethernet0/0
 ip address 10.0.0.1 255.255.255.252
 half-duplex

interface Ethernet0/1
 ip address 172.16.0.2 255.255.255.0
 half-duplex

ip route 172.16.1.0 255.255.255.0 172.16.0.1 tag 20
ip route 172.16.2.0 255.255.255.0 172.16.0.1 tag 20

router eigrp 100
 redistribute static route-map MyRouteMap
 network 10.0.0.0 0.0.0.3
 no auto-summary

route-map MyRouteMap permit 10
 match tag 20

Router R2:

interface Ethernet0/0
 ip address 10.0.0.2 255.255.255.252
 half-duplex

interface Ethernet0/1
 ip address 10.0.0.5 255.255.255.252
 half-duplex

router eigrp 100
 network 10.0.0.0 0.0.0.3
 no auto-summary

router bgp 65000
 no synchronization
 bgp log-neighbor-changes
 redistribute eigrp 100 route-map MyRouteMap
 neighbor 10.0.0.6 remote-as 65000
 neighbor 10.0.0.6 next-hop-self
 no auto-summary

route-map MyRouteMap permit 10
 match tag 20

R2#show ip route 172.16.1.0
Routing entry for 172.16.1.0/24
  Known via "eigrp 100", distance 170, metric 307200
  Tag 20, type external

Router R3:

interface Ethernet0/1
 ip address 10.0.0.6 255.255.255.252
 half-duplex

router bgp 65000
 no synchronization
 bgp log-neighbor-changes
 neighbor 10.0.0.5 remote-as 65000
 no auto-summary

R3#show ip bgp 172.16.1.0
BGP routing table entry for 172.16.1.0/24, version 3
Paths: (1 available, best #1, table Default-IP-Routing-Table)
  Not advertised to any peer
  Local
    10.0.0.5 from 10.0.0.5 (10.0.0.2)
      Origin incomplete, metric 307200, localpref 100, valid, internal, best

I hope this is what you were asking for.

Cisco VPNv4 Over eMPBGP – Comprehensive Guide

Wasn't able to resolve my issue.

Best Answer

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Cisco – route tags and policy routing alternatives – traffic engineering question

Cisco VPNv4 Over eMPBGP – Comprehensive Guide

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