In this blog we will discuss about one important knob in JunOS i.e vrf-table-label.
Vrf-table-label is useful for 2 purposes in Junos
- Save label space
- Perform 2 lookup on packet
So let’s understand it more. We will start with 1st point above
Junos by default allocates same VPN Label to prefixes recieved from one CE Interface. So for example if you have 2 CEs connected via 2 different interfaces and they are in same VPN on PE then Junos will allocate 2 different VPN labels to the prefixes recieved. In Cisco this is different where VPN label is allocated on per prefix which according to some is not optimal but we are not comparing anything here.
Currently in our configuration vrf-table-label is not configured. If you see below, we have 2 CEs connected to Juniper M320 PE1 via 2 different interfaces and we have Ebgp relationship between them and we are receiving some routes over it.
PE1-re1> show route 10.203.20.6
MVPN-1.inet.0: 46 destinations, 77 routes (46 active, 0 holddown, 0 hidden)
Restart Complete
+ = Active Route, – = Last Active, * = Both
10.203.20.4/30 *[Direct/0] 3d 00:21:55
> via ge-0/3/2.20
PE1-re1> show route 10.203.12.2
MVPN-1.inet.0: 46 destinations, 77 routes (46 active, 0 holddown, 0 hidden)
Restart Complete
+ = Active Route, – = Last Active, * = Both
10.203.12.0/30 *[Direct/0] 00:10:26
> via so-1/0/0.12
PE1-re1> show route receive-protocol bgp 10.203.12.2 table MVPN-1.inet.0
MVPN-1.inet.0: 46 destinations, 77 routes (46 active, 0 holddown, 0 hidden)
Restart Complete
Prefix Nexthop MED Lclpref AS path
* 10.1.225.128/32 10.203.12.2 65012 I
10.203.12.0/30 10.203.12.2 65012 I
PE1-re1> show route receive-protocol bgp 10.203.20.6 table MVPN-1.inet.0
MVPN-1.inet.0: 46 destinations, 77 routes (46 active, 0 holddown, 0 hidden)
Restart Complete
Prefix Nexthop MED Lclpref AS path
* 10.0.233.0/30 10.203.20.6 65020 I
Now if we look at the VPN label which is being tagged by this PE1 for the routes received by CE, we can see that Junos is allocating separate VPN Labels to both of the routes which is what I mentioned earlier.
PE1-re1> show route advertising-protocol bgp 10.198.123.236 10.0.233.0/30 extensive
MVPN-1.inet.0: 46 destinations, 77 routes (46 active, 0 holddown, 0 hidden)
Restart Complete
* 10.0.233.0/30 (2 entries, 1 announced)
BGP group mvpn-rr type Internal
Route Distinguisher: 10.198.123.203:32764
VPN Label: 300448
Nexthop: Self
Flags: Nexthop Change
Localpref: 100
AS path: [65004] 65020 I
Communities: target:65000:321 src-as:65004:0 rt-import:10.198.123.203:16
PE1-re1> show route advertising-protocol bgp 10.198.123.236 10.203.12.0/30 extensive
MVPN-1.inet.0: 46 destinations, 77 routes (46 active, 0 holddown, 0 hidden)
Restart Complete
* 10.203.12.0/30 (2 entries, 1 announced)
BGP group mvpn-rr type Internal
Route Distinguisher: 10.198.123.203:32764
VPN Label: 300480
Nexthop: Self
Flags: Nexthop Change
Localpref: 100
AS path: [65004] I
Communities: target:65000:321 src-as:65004:0 rt-import:10.198.123.203:16
Now if we configure the vrf-table-label under routing instance on PE1, we can see the difference.
[edit routing-instances MVPN-1]
PE1-re1# set vrf-table-label
edit routing-instances MVPN-1]
PE1-re1# commit
re1:
configuration check succeeds
re0:
commit complete
re1:
commit complete
See the difference below, now only one VPN label is being allocated for the whole VRF and this really saves the label space.
PE1-re1> show route advertising-protocol bgp 10.198.123.236 10.203.12.0/30 extensive
MVPN-1.inet.0: 46 destinations, 77 routes (46 active, 0 holddown, 0 hidden)
Restart Complete
* 10.203.12.0/30 (2 entries, 1 announced)
BGP group mvpn-rr type Internal
Route Distinguisher: 10.198.123.203:32764
VPN Label: 39
Nexthop: Self
Flags: Nexthop Change
Localpref: 100
AS path: [65004] I
Communities: target:65000:321 src-as:65004:0 rt-import:10.198.123.203:16
PE1-re1> show route advertising-protocol bgp 10.198.123.236 10.0.233.0/30 extensive
MVPN-1.inet.0: 46 destinations, 77 routes (46 active, 0 holddown, 0 hidden)
Restart Complete
* 10.0.233.0/30 (2 entries, 1 announced)
BGP group mvpn-rr type Internal
Route Distinguisher: 10.198.123.203:32764
VPN Label: 39
Nexthop: Self
Flags: Nexthop Change
Localpref: 100
AS path: [65004] 65020 I
Communities: target:65000:321 src-as:65004:0 rt-import:10.198.123.203:16
So this completes one part. Now moving over to 2nd part.
Junos by default looks at the incoming MPLS packet, Pops the label and sends the underlying packet to CE without looking at IP packet at all. This situation is fine in case you have PE connected to CE via P2P links like Serial links however if you have Broadcast medium like Ethernet in between then router can’t just send the packet like this without first building the frame and to build frame it needs to do ARP lookup to get the MAC Address of the CE. So it needs to do extra lookup apart from MPLS lookup.
Vrf-table-label actually allows the router to do 2 lookups. The first lookup is done on the VPN label to determine which VRF table to refer to, and the second lookup is done on the IP header to determine how to forward packets to the correct end hosts on the shared medium. This can be useful for number of applications like ingress firewall filters, CoS etc. Now a days VT interface (tunnel-pic) is also used to do the same however if router doesn’t support tunnel-pic then vrf-table-label can be used in its place to do the same thing. With VTL, lsi interface is created which allows it to handle the first lookup before a second ARP/IP lookup is carried out through the PFE.
Lets rollback the changes we did above and come back to same situation where unique label is assigned per CE port.
VPN Label 300560 is assigned for the route by PE1 and when mpls table is checked for that particular label we can see action is Pop plus to send the packet directly to interface.
PE1-re1> show route advertising-protocol bgp 10.198.123.236 10.203.12.0/30 extensive
MVPN-1.inet.0: 46 destinations, 77 routes (46 active, 0 holddown, 0 hidden)
Restart Complete
* 10.203.12.0/30 (2 entries, 1 announced)
BGP group mvpn-rr type Internal
Route Distinguisher: 10.198.123.203:32764
VPN Label: 300560
Nexthop: Self
Flags: Nexthop Change
Localpref: 100
AS path: [65004] I
Communities: target:65000:321 src-as:65004:0 rt-import:10.198.123.203:16
PE1-re1> show route table mpls.0 label 300560
mpls.0: 57 destinations, 57 routes (57 active, 0 holddown, 0 hidden)
Restart Complete
+ = Active Route, – = Last Active, * = Both
300560 *[VPN/170] 00:00:41
> via so-1/0/0.12, Pop
If we enable the vrf-table-label now and check the same route and corresponding label. Lets see what we see.
PE1-re1> show route advertising-protocol bgp 10.198.123.236 10.203.12.0/30 extensive
MVPN-1.inet.0: 46 destinations, 77 routes (46 active, 0 holddown, 0 hidden)
Restart Complete
* 10.203.12.0/30 (2 entries, 1 announced)
BGP group mvpn-rr type Internal
Route Distinguisher: 10.198.123.203:32764
VPN Label: 40
Nexthop: Self
Flags: Nexthop Change
Localpref: 100
AS path: [65004] I
Communities: target:65000:321 src-as:65004:0 rt-import:10.198.123.203:16
PE1-re1> show route table mpls.0 label 40
mpls.0: 53 destinations, 53 routes (53 active, 0 holddown, 0 hidden)
Restart Complete
+ = Active Route, – = Last Active, * = Both
40 *[VPN/0] 00:00:12
to table MVPN-1.inet.0, Pop
So we can see, label 40 is basically pointing to routing-table now and not to interface as in our previous case. You can see the corresponding LSI interface allocated by looking at following command
PE1-re1> show route instance MVPN-1 detail
MVPN-1:
Router ID: 10.14.233.1
Type: vrf State: Active
Restart State: Complete Path selection timeout: 300
Interfaces:
lsi.24
so-1/0/0.12
ge-0/3/3.50
ge-0/3/2.20
vt-1/2/0.20
Route-distinguisher: 10.198.123.203:32764
Vrf-import: [ __vrf-import-MVPN-1-internal__ ]
Vrf-export: [ __vrf-export-MVPN-1-internal__ ]
Vrf-import-target: [ target:65000:321 ]
Vrf-export-target: [ target:65000:321 ]
Fast-reroute-priority: low
Tables:
MVPN-1.inet.0 : 77 routes (46 active, 0 holddown, 0 hidden)
Restart Complete
MVPN-1.inet.1 : 11 routes (9 active, 0 holddown, 0 hidden)
Restart Complete
MVPN-1.mvpn.0 : 77 routes (42 active, 7 holddown, 0 hidden)
Restart Complete
Ok so that’s all. I hope you liked the blog and I was able to resolve some of your confusion on this command. If you still have any queries, please let me know and I would be happy to discuss.
Regards
Mohit Mittal
networkzblogger.wordpress.com 