<nav id="topnav">
 <div class="container">
        <a  routerLinkActive="active" tabindex="1" class="skip-link" href="#skipContent">Skip to content</a>
        <a routerLinkActive="active" tabindex="2" routerLink="/page1a">Page1-a</a>
        <a routerLinkActive="active" tabindex="2" routerLink="/page1b">Page1-b</a>
        <a routerLinkActive="active" tabindex="2" routerLink="/page1c">Page1-c</a>
 </div>
<nav>

import { Component, OnInit } from '@angular/core';
@Component({
  selector: 'app-topNav',
  templateUrl: './topNav.component.html',
  styleUrls: ['./topNav.component.scss']
})
export class topNavComponent implements OnInit {

  constructor() { }

  ngOnInit() {
  }

}

<nav id="main-navigation">

 <div *ngIf="section == 'home'; else homeInactive">
  <a id="Home" routerLinkActive="inactive"
    routerLink="/page1a">
  </a>
 </div>

<ng-template #homeInactive>
  <a id="Home"
    routerLinkActive="active" routerLink="/page1a">
  </a>
</ng-template>

 <a id="leftTab2" routerLinkActive="inactive"
  routerLink="/leftTab2">
 </a>

 <a id="leftTab3" routerLinkActive="inactive" routerLink="/leftTab3">
 </a>
</nav>

@Component({
  selector: 'app-leftNav',
  templateUrl: './leftNav.component.html',
  styleUrls: ['./leftNav.component.scss']
})
export class leftNavComponent implements OnInit {

  @Input() section: string;
  constructor() { }

  ngOnInit() {
  }

}