MPLS CHAPTER 4

MPLS CHAPTER 4
QUESTIONS:
12a. LSRs that get a packet with an unknown label what it will do ?
12b. What is PHP ?
13. What is relationsip between RIB/FIB , LIB/LFIB CEF ?
14. Which is preferred by CEF: MPLS or IPv4 forwarding ?
15. What are different type of labels used in MPLS ?
ANSWERS:

------->(LSR_ROUTER)------>
12a. LSRs that get a packet with an unknown label what it will do ?
12b. What is PHP ?
14. Which is preferred by CEF: MPLS or IPv4 forwarding ?
When pkt arrieves to router according to its label it forward pkt to next-hop
if it don't have label information it will process according to IGP reachability.

incoming_pkt      outgoing_decision(data plan)
IPv4                 IPv4 (FIB)
IPv4                 MPLS (LFIB)
MPLS              MPLS (LFIB)
MPLS              IPv4  (FIB)

Penultimate Hop popping (PHP)
. Penultimate means netxt to last
. Normally last hop must ....
- Lookup MPLS Label
- Pop MPLS Label
- Lookup IPv4 destination
. PHP avoids extra lookup on last hop
. Accomplished through implicit NULL label
advertisement for connected prefixes.



13. What is relationsip between RIB/FIB , LIB/LFIB CEF ?

Check the diagram for : RIB/FIB & LIB/LFIB

either it is label/unlabelled pkt forwarding decision taken place with cef entry.
diagram:

let us check some output on R2 on the given diagram:
for RIB:
sh ip ospf 1 rib
for FIB
R2#sh ip route ospf 1
     1.0.0.0/32 is subnetted, 1 subnets
O       1.1.1.1 [110/11] via 10.1.12.1, 00:45:34, FastEthernet0/0
     3.0.0.0/32 is subnetted, 1 subnets
O       3.3.3.3 [110/11] via 10.1.23.3, 00:23:18, FastEthernet0/1

for LIB
R2#sh mpls ldp bindings
  tib entry: 1.1.1.1/32, rev 2
        local binding:  tag: 18
        remote binding: tsr: 1.1.1.1:0, tag: imp-null
        remote binding: tsr: 3.3.3.3:0, tag: 22
  tib entry: 2.2.2.2/32, rev 4
        local binding:  tag: imp-null
        remote binding: tsr: 1.1.1.1:0, tag: 500
        remote binding: tsr: 3.3.3.3:0, tag: 23
  tib entry: 3.3.3.3/32, rev 6
        local binding:  tag: 19
        remote binding: tsr: 1.1.1.1:0, tag: 502
        remote binding: tsr: 3.3.3.3:0, tag: imp-null
  tib entry: 10.1.12.0/24, rev 8
        local binding:  tag: imp-null
        remote binding: tsr: 1.1.1.1:0, tag: imp-null
        remote binding: tsr: 3.3.3.3:0, tag: 24
  tib entry: 10.1.23.0/24, rev 10
        local binding:  tag: imp-null
        remote binding: tsr: 1.1.1.1:0, tag: 501
        remote binding: tsr: 3.3.3.3:0, tag: imp-null

for LFIB 

R2#sh mpls forwarding-table
Local  Outgoing    Prefix            Bytes tag  Outgoing   Next Hop
tag    tag or VC   or Tunnel Id      switched   interface
18     Pop tag     1.1.1.1/32        0          Fa0/0      10.1.12.1
19     Pop tag     3.3.3.3/32        0          Fa0/1      10.1.23.3

for particular CEF entry:
R2#sh ip cef 1.1.1.1 de
R2#sh ip cef 1.1.1.1 detail
1.1.1.1/32, version 20, epoch 0, cached adjacency 10.1.12.1
0 packets, 0 bytes
  tag information set
    local tag: 18
  via 10.1.12.1, FastEthernet0/0, 0 dependencies
    next hop 10.1.12.1, FastEthernet0/0
    valid cached adjacency
    tag rewrite with Fa0/0, 10.1.12.1, tags imposed: {}

15. What are different type of labels used in MPLS ?

Pop—The top label is removed. The packet is forwarded with the remaining label stack or as an unlabeled packet.
Swap—The top label is removed and replaced with a new label.
Push—The top label is replaced with a new label (swapped), and one or more labels are added (pushed) on top of the swapped label.
Untagged/No Label—The stack is removed, and the packet is forwarded unlabeled.
Aggregate—The label stack is removed, and an IP lookup is done on the IP packet.